Elongate webs, for example webs of thermoplastic synthetic resin, and especially synthetic resin foils, are generally stored, marketed and handled in rolls or coils which may have lengths of hundreds of meters and, depending upon the thickness of the foil, corresponding diameters and mass.
The foil is usually made on a foil-blowing machine in which the extruded tubular strand of the synthetic resin material is blown and then cooled to form a tubular foil which can be supplied, slit or rolled without further modification after the blown foil has been flattened, e.g. between a pair of rollers.
The rolling of the foil is usually effected in a coiling portion of the apparatus by passing the foil over a driven drum (contact drum) which rotates the coil as the web is wound up thereon. The web can be wound on mandrels which can be lifted from the coiling machine to deliver it to bag-making machinery or the like.
A conventional apparatus which winds continuous webs into a series of coils has been described in published European patent application No. 0 017 177 published Oct. 15, 1980. In this device, a sensor is provided for the pressure with which the coil bears against the contact drum. The sensor is a force-sensing device on the contact drum assembly which responds to the pressure applied to the contact drum and which is then utilized to control, via a fluid cylinder, a mechanism which regulates the force with which the coil presses on this drum, the coil and the mandrel upon which it is wound being supported by a pair of swingable arms so that the coil can press against the feed drum.
The problem with this system is the insufficient precision of the pressure control. The sensor registers not only the pressing forces but all kinds of perturbations which result from the drive of the contact drum. These include vibrations, torque fluctuations and the like. The irregularities in the coil or roll, unevenness in the foil which passes between the drum and the coil as it is wound onto the coil, and the like, provide both periodic and aperiodic disturbances in the force measurement.
While a filtering of periodic disturbances might, at first blush, be a practical solution to the lack of precision, in practice it is found that the periodic fluctuations are not predictable and, in any event, such filtration cannot eliminate the fluctuations resulting from aperiodic phenomena.
As a consequence, the device may respond erroneously or may so provide pressure control that the actual force with which the coil bears against the contact drum may vary with greater fluctuations than might be the case in the absence of the control. As a consequence, it may be necessary to provide additional control means to lift the coil away from the contact drum and for driving the coil independently of the contact drum to prevent any retardation of coiling which may result in a backing up of the web fed to the coil. Additional controls of this type are complex and introduce additional maintenance and cost factors.
In practice, therefore, the earlier system has not found application.